Transmission system



May 12, 1942,

R. H. HERRICK TRANSMISSION SYSTEM 3 Sheets-Shet 1 Filed Aug. 14, 1959 INVENTOR. ROSWELL H HERRICK 3mm,- JM +11% ATTORNEYS May 12, 1942. R. H. HERRICK I TRANSMISSION SYSTEM Filed Aug. 14, 1939 3 Sheets-Sheet 2 a QM INVENTOR.

RQSWELL H HERRICK h ATTORNEYS May 12,1942- R. H. HERRICK TRANSMISSION SYSTEM Filed Aug. 14, 1939 3 Sheets-Sheet 3 INVENTOR.

ELL- H. HER RICK e4u ,-MML4 ATTORNEYS,

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, average line conditions.

inarnplification of incoming and outgoing signal Patented May 12, 1942 TRANSMISSION SYSTEM Roswell H. Herrick, ak Park, Ill., assignor to Associated Electric laboratories, Inc., Chicago, 11]., a corporation of Delaware Application August 14, 1939, Serial No. 290,090

8 Claims.

The present invention relates generally to improvements in signal current transmission systems of the type in which signal controlled switching circuits are provided for partially or completely-blocking, under certain conditions, certain of the signal current channels included therein and, more particularly, to improvements in telephone substation circuits having incorporated therein coupled signal current channels for the transmission of signal currents.

In the usual telephone substation circuit a hybrid system or antiside tone impedance network is provided for preventing signal currents developed during operation of the transmitter from being transmitted to the receiver and for similarly preventing signal currents incoming over the line extending to the substation from being transmitted to the transmitter for reproduction. In this type of arrangement one of the factors which determines the efliciency of the side tone suppression is the impedance of the talking circuit established by way of two connected subscribers lines. This impedance is not the same for any two diiferent established connections and, accordingly, the hybrid system of each substation is usually balanced to provide maximum side tone suppression efilciency for In installations wherecurrents is required, as, for example, in executive loud speaking sets, the conditions of unbalance introduced in the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. This is particularly true in substation installations provided'in an exchange area where the subscribers lines are of widely different lengths. In order completely to obviate or to minimize the singing which may result due to unbalance of the substation circuit occasioned by unfavorable line conditions, signal controlled switching means may be provided in the substation circuit for selectively blocking the channels when not in use. In the usual arrangement of this character the signal channels are completely blocked when notin use. More particularly, during intervals when the transmitting means of the substation is being. used to transmit outgoing signal currents, the incoming signal current channel is rendered completely inactive. Conversely, during those periods when signal currents are incoming to the substation, the transmitting or outgoing signal current channel is incoming and outgoing rendered'completely inactive. An arrangement define a space current of this character, while satisfactory in operation, prevents any interruption of a speech train being transmitted in one direction in response to signal currents transmitted in the opposite direction. In other words, conversation breakins are positively prevented.

It is an object of the present invention to provide improved telephone substation apparatus of the character described wherein the channels are preferably only partially blocked.- when not in use, the blocking of the signal current transmitting and receiving channels is accomplished in a simple and reliable manner, and wherein the control circuits are arranged to be exceed ingly fast in operation so that speech clipping is minimized.

It is another object of the invention to pro vide an improved transmission system particularly suited for use in telephone substation circuits of the character described wherein the control or channel blocking circuit is arranged in an improved manner to utilize gaseous discharge tubes of the three-electrode type, whereby the circuit is rendered exceedingly fast in operation the hybrid system-to the associated line and a second channel for transmitting incoming signal currents from the hybrid system to the receiving means. In accordance with the presentinvention there is associated with each channel a control circuit comprising agaseous discharge tube including an anode and a cathode separated to path and a startelectrode for controlling the flow of current over the space current path, together with means controlled by the flow of current over this' path for changing the transmission efiiciency of at least one of the I. v I rectifying means 1s provided-for coupling the input elecchannels. Apparatus comprising trodes of each of the'gaseous discharge tubes to the associated channel so that a predetermined start potential is impressed upon thestart' electrode of the tube when signal current traverses thissignal. as is well known, a ftubeoi' this type has an operating characteristic such that when a space discharge is established between the anode and cathode thereof through the application of a start potential to its start electrode, the start electrode loses control and the arc can only be extinguished by breaking the the anode and cathode of the associated dis-- charge tube when the flow of signal currents through the associated channel is arrested, thereby to arrest the flow of current over or through the space current path and to restore the control of this path to the start electrode. Two different embodiments of the last-mentioned arc extinguishing means are disclosed. In one of these embodiments a slow-to-release relay is provided in the load or output circuit of the rectifying means, which relay functions momentarily to interrupt the anode circuit of the associated gaseous. discharge tube when the input signal voltage derived from the associated channel and mpressed upon the input circuit of the rectifying means is removed. In the other embodiment there is locally provided a source of alternating current which is utilized as they anode current source for each of the two discharge tubes. With the latter arrangement, when the start electrode of one of the tubes is biased to the start potential the tube functions as a half wave rectifier so that direct voltages are developed in the output circuit thereof which are utilized to perform the desired channel control functions.

Further features of the invention pertain to the particular arrangement of the circuit elements, whereby the above and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization'and method of operation, together with further objects and advan-- tages thereof will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figure 1 illustrates a substation circuit having incorporated therein certain of the features of the invention as briefly outlined above; Fig. 1a illustrates a slightly different arrangement of certain of the circuit elements shown in Fig. 1; Fig. 2 illustrates a modified form of the substation circuit shown in Fig. 1; and Fig. 3 illustrates a different embodiment of the improved transmission system as incorporated in a substation circuit;

Referring now more particularly to Fig. 1 of the drawings, the substation circuit there illustrated is adapted to be connected to a telephone line terminating at the terminals I which may, for example, form a part of a conventional automatic or manual telephone system, in which case, the distant end thereof will terminate in a line circuit the character of which is determined by the character of the exchange. Briefly described, the circuit comprises a transmitter or microphone IM and receiving means in the form of a loud speaker I02 which are adapted respectively to be coupled to an antisidez tone impedance network including a hybrid coil I03 and a balancing circuit I04 by means of signal transmission channels I05 and I06. The outgoing signal current channel I05 comprises" a vacuum tube amplifier I08 including three stages of amplificationrespectively including the amplifier tubes I09, H0 and III, the first of which is adapted to receive signal currents from the microphone IOI and the last of which is arranged to deliver its output to the hybrid system I03. More particularly, the microphone IOI is coupled to the input electrodes of the first amplifier tube I09 through a coupling network which includes a condenser H2 and an adjustable voltage dividing or signal level control resistor II3. The output electrodes of the tube I09 are coupled to the input electrodes of the succeeding amplifier tube IIO through a, resistance capacitance coupling network which includes a pair of resistors H4 and H5 and a coupling condenser H6. Similarly the output electrodes of the second amplifier tube H0 are coupled to the input electrodes of the final amplifier tube I II through a resistance capacitance coupling network which includes a resistor III, a voltage dividing or signal level adjusting resistor H8 and a coupling condenser II9. In order to bias the control electrode of the tube I09 to the proper negative potential with respect to the associated cathode, there is provided in the cathode leg of the output circuit of this tube a pair of series connected cathode biasing resistors I20 and I2I which are shunted by a signal current by-pass condenser I22. The control grid of the tube I I0 is similarly biased to the proper negative potential with respect to its associated cathode by means of a cathode biasing circuit comprising a resistor I23 shunted by a signal current by-pass condenser I24. A similar cathode biasing network comprising a resistor I25 shunted by a by-pass condenser I28 is included in the cathode leg of the output circuit of the final amplifier tube III. Anode potentials are supplied to the anodes of the tubes I09, H0 and III from a source of anode current, not shown, but having its positive terminal connected to the terminal marked +B, over circuits which respectively include the resistors H4, H1 and certain of the windings of the hybrid coil I03. Screen current source over paths respectively including filter networks, that leading to the screen electrode of the tube I09 including a resistor I21 and a signal current by-pass condenser I28 and that extending to the screen electrode including a resistor I29 and a by-pass condenser I30.

The incoming signal current transmission channel I06 has included therein an amplifier I35-which includes two stages of amplification respectivelycomprising the amplifier tubes I36 and I31. More specifically, the incoming signal current terminals of the hybrid coil I03 are coupled to the input electrodes of the first amplifier tube I36 through a coupling network which includes a transformer I38 and an adjustable voltage dividing or signal level control resistor I39.

The output electrodes of this tube are coupled to the input electrodes of the second amplifier tube I'3'l through a resistance capacitance couplingnetwork which comprises a resistor I40, a coupling condenser I and an adjustable voltage dividing or signal level control resistor I43. The output electrodes of the second amplifier tube I31 are, in turn, coupled to the voice or signal current coil of the loud speaker I02 through a coupling transformer I44 and a T connected signal current level control resistance network comprising a pair'of series resistors I45 and I45 and a shunt resistor I46. In order to bias the contrpl electrode of the tube I3! to the proper negative potential with respect to the associated cathode, there is provided in the cathode leg of the output circuit of this tube a biasing network which includes a resistor I01 shunted by a signal current by-pass condenser Ilii. control electrode of the first amplifier tube I06 is effected through the provision of a cathode biasing network which includes a pair of resistors I 49 and I50 which are shunted by a signal current by-pass condenser II. Anode potentials Control of the bias impressed upon the,

trodes of the rectifier tube I65 are'included in an 4 output circuit which comprises'the two resistors are impressed upon the anode electrodes of the two tubes I36 and I31 from the source of anode current over circuits respectively including a re-" sistor I and the primary winding of the coupling transformer I. Screen potential is impressed upon the screen electrode of the tube I36 from the source of anode current over a path which includes a filter network comprising a resistor I52 and a by-pass condenser I53. Screen potential for the screen anode of the tube I31 is derived in a manner more fully explained subsequently.

In order to control the gains of the amplifiers I08 and I35, thereby to vary the signal current transmission efficiencies of the channels I05 and I06 in accordance with signal current flow therethrough there is provided a control circuit I55 which includes two control units, the first of which operates in response to signal currents traversing the channel I05 and the second of which operates in response to signal currents traversing the channel I06. More particularly, the first unit of the control circuit comprises a gaseous discharge tube I56 of the three-electrode type having an anode I51 and a cathode I56 separated to define a space current path and a control or start electrode I56 interposed therebetween to control the fiow of current over the same space current path. The start electrode I59 is normally biased negatively with respect to the cathode I56 over a path which serially includes a resistor I60, a source of bias voltage I6I, a resistor I62 and a resistor I60. The space cur- .rent path of the discharge tube I56 is included in the output circuit of this tube in series with the source of anode potential mentioned above and the resistors I and I 63, whereby control voltages are developed across these resistors when a discharge through the tube is established, which voltages are utilized to control the final amplifier tube III included in the channel I05 and the first amplifier tube I06 included in the channel I06. More particularly, the voltage developed across the two series connected resistors I50 and I66 in response'to space current traversing the tube I 56 is utilized as the screen potential for the amplifier tube III, whereby this amplifier tube is only rendered operative to transmit signal currents therethrough in response to a flow of current over the space current path of the tube I56. Since the resistor I50 is also included in the cathode biasing circuit of the amplifier tube I66, the voltage across this resistor also functions'to control the signal current gain of the amplifier I85. The two serially connected-resistors I50 and I63 are heavily bypassedfor signal currents through the provision of a low impedance condenser I60. In order to impress a start potential upon the start grid I50 of the discharge tube I56 when signal currents traverse the channel I05 a rectifying unit is provided which includes a three-electrode vacuum tube I65 having its input electrodes coupled to the channel I05 at a point following the ampli g and I1! of this tube circuit which also Includes the resistor I2I, a

start electrode I10 an armature I 69 which amplifier tube I06 included in I50 and I62 and the source of anode current connected to the terminal marked +-B. In order momentarily to interrupt the output circuit of the gaseous discharge tube I56 when signal current flow over the is provided a slow-to-release relay I having its winding included in the output circuit of the tube I to be energized by space current traversing this tube.- This relay is provided with is arranged to interrupt the output circuit of the discharge tube I56.

The second control trol circuit I55, namely, that associated with and responsive to signal currents traversing the channel I06 is essentially identical to that Just described and provided in association with the channel I05. More particularly. the second control unit comprises a three-electrode gaseous discharge tube I10 having an anode Ill and acathode I12 separated to define a space current path and a start electrode I10 interposed in this path for the purpose of controlling the initial flow of current thereover. The output electrodes I are included in an output second resistor I10 and the source of anode current connected to the terminal marked +13. The voltage developed across these two series connected resistors when space'current flows through the tube I10 is utilized as the operating potential for the screen I01 included in the channel I06, these two resistors being heavily byfor signal currents by a low impedance condeer I15 connected in shunt therewith. Since e resistor I2I forms a part of the :athode biasing circuit for the first the outgoing signal current channel I05, the voltage across this resistor serves to determine'the gain of the indicated amplifier tube and thus the signal current transmission efliciency of the channel I05. The of the discharge tube I10 is normally biased negatively with respect to its associated cathode over a path which includes a resistor I16, a source of bias voltage I11, a second resistor I10 shunted by a signal current by-pass condenser I10 and the resistor I10. For the purpose of impressing a start potential upon the start electrode I10 when signal currents traverse the channel I00, a rectifier is provided which comprises a three-eiectrode'vacuum tube I00 having its input electrodes coupled to the channel I06 at a point tube I00 over a circuit denser IOI. erate as as battery 102 connected between the input electrodes thereof. The output electrodes of the tube I00 are included in an output circuit which serially comprises the resistors Ill and I10. In

order to control the deionization of the discharge tube I10 there is provided a slow-t'o-release relay I00 having its operating winding included in the output circuit of the tube I00 and provided with an armature I00 which is operative to make and break the output circuit of the discharge tube I10.

Referring now more particularly to the operachannel I05 is arrested, there unit embodied in the conelectrode of the amplifier tube.

following the amplifier V tion of the system illustrated in Fig. 1, it is pointed out that in order to condition the signal current channels .for signal current transmission, the cathode heater circuit, not shown, for the various tubes illustrated, is first closed. With the apparatus in this condition the amplifier stages respectively comprising the tubes I09, H and I36 are all biased to operate at normalgain. Since, however, the two discharge tubes I56 and III! are at this time in a deionized condition, no screen potentials are available for the screen electrodes of the final amplifier tubes III and I 31 respectively included in the amplifiers I06 and I35. Thus, both channels are effectively blocked so that no circulation of energy around the system can occur. If, with the apparatus in this condition, a user of the substation circuit speaks into the microphone IQI, the resulting signal voltage is impressed upon the input electrodes of the amplifier tube I09 and the amplified output voltage is, in turn, impressed upon the input electrodes of the tube H6. As further amplified by the tube I ID the signal voltage is impressed across the input electrodes of the amplifier stage I I I and also through the condenser I66 across the input electrodes of the rectifier tube I65. During alternate hali cycles of this voltage the bias potential impressed upon the control electrode of the tube I65 by the battery I61 is overcome so that space current flows through this tube over a circuit which includes the winding of the relay I68, the resistor I52 and the resistor I50. As indicated above, during signal current transmission through the channel I05, due to the magnitude of the bias impressed across the control electrodes thereof by the battery I81, the vacuum tube I85 functions as a half wave rectifier. Accordingly, a pulsating direct current traverses the space current path of this tube and the resistors I62 and IE0. Due to the smoothing action of the condenser I62 shunting the resistor I62, however, the voltage across this resistordoes not fall to a zero value during alternate half cycles of signal current but is maintained at a value suficient to permit ionization of the discharge tube Itt. The resulting voltage developed across thelresistor lei is impressed across the input electrodes I58 and its of the discharge tube I56 in opposition to the negative bias voltage afforded by the C battery I6I. This voltage is of sufiicient magnitude to lower the potential of the start grid ISQ to a point where a discharge through this tube occurs. When the tube I56 is ionized in this manner, current is caused to traverse the space current path thereof; this current also flowing through the series connected resistors IW and I83. The last-mentioned resistor has a resistance value substantially greater than that of the resistor Hill and also substantially greater than the resistance of the space current path through the tube I56. Accordingly,'

a substantial portion of the available voltage of v the anode current supply source appears as a voltage drop across the resistor I63. Thisvoltagedrop, as augmented by the voltage drop across the resistor I80, is impressed. as an operating potential upon the screen electrode of the final amplifier tube III included in the channel I05, thereby to render the tube II-I operative to transmit the generated signal currents therethrough and through the hybrid coil I" to the line terminating at the terminals IIII. As indicated above, the operations just described all 0c cur during the first cycle of the signal current developed in response to the sound waves impinging upon the diaphragm of the microphone IIlI.

Accordingly, no substantial clipping of the first I syllable of the speech represented by the sound waves occurs. Simultaneously with the activation of the amplifier tube III the first amplifier tube I 36 of the channel IDS is either biased to have a I50 and flowing through the space current paths of the tubes I56 and its causes a voltage to be developed across this resistor which is impressed through the resistor use upon the cathode of the tube I36 as a positive bias. This bias voltage has the effect of lowering the gain of the tube I36 and if of sufficiently large magnitude may serve to bias this tube to a point beyond anode current cut-oii such that the channel I06 is completely blocked in the first amplifier stage thereof. Alternatively, if the resistance value of the resistor I58 is chosen so that the voltage developed thereacross by the space currents flowing therethrough does not exceed, the anode current cutoff value of the tube I36, this tube will pass incoming signal currents to the succeeding amplifier tube I3? at a substantially reduced gain. By virtue of the arrangement just described if the antiside tone network comprising the hybrid coil Hi3 and the balancing circuit IM are greatly unbalanced'due to the condition of the lines connected to the terminals MU, such that signal currents are transmitted from the channel I05 through this network to the input side of the channel I06, the partial or complete blocking of the latter channel in the first amplifier stage comprising the tube I38 prevents the signal currents from being fed through the channel to initiate operation of the second control unit comprising the rectifier tube I and the discharge tube IIIl.

From the foregoing explanation it will be apparent that the current traversing the space current path of the tube I65 as a result of signal voltage impressed upon the input electrodes of this tube also traverses the operating winding of the slow-to-release relay I88. Shortly following the first few cycles of this voltage, the relay I88 operates to break, at its armature I69 and the associated back contact, the previously traced output circuit including the space current path of the discharge tube I56. This circuit is recompleted at the armature I 69 and its associated working contact when the armature W9 is moved to its attracted position. During the interval required for operation of the armature I88 from its retracted position to its attracted position the output circuit of the tube IN is held open. Accordingly, the fiow of space current through this tube and through the resistors I60 and I63 is momentarily interrupted, causing the final amplifier tube III of the channel it momentarily to be rendered inoperative and the bias voltage impressed across the input electrodes of the first amplifier tube I38 included in the channel I08 momentarily to be returned to the normal value.

tube I66. Accordingly, immediately the output circuits of the discharge tube I58 is recompleted at the armature I69 and its associated front, contact, space current again flows through the tube I56, thereby to render the final amplifier tube III operative and partially or completely toblock the signal transmission channel I06. During the 2,282,405 interval when the armature I69 is operating between its two positions no signal current transmission through the channel I65 can occur. Accordingly, a. very slight clipping of the intermediate syllables of the speech train may occur during this switching interval. Such clipping is of no consequence, however, since the listener, upon hearing the first syllables of the speech train and the syllables following the interval during which clipping may occur, can readily ascertain. by im terpolation, the syllable or partial syllable which i is cut off during the switching operation.

-When the flow of signal currents through the channel I65 is arrested, the flow of current through the space current path of the rectifier tube I65 is reduced to zero or to a negligible value. Accordingly, the potential impressed upon the start electrode I59 of the discharge tube I56 is increased in a negative sense to a value which will prevent further current fiow through this tube after the tube is deionized. As explained above, the characterof the discharge tube I56 positions the output circuit of the discharge tube I56 is interrupted in an obvious manner so that no voltage is available for sustaining the discharge through the tube. Accordingly, the tube I56 is deionized, at which time the control of the space current pathin the tube is restored to the negatively biased start electrode I59. Due to the negative potential impressed upon this electrode by the battery I 6I, ionization-of the tube is prevented when the armature I69 is moved into engagement with its associated back contact to again impress the voltage of the anode current source across the anode I51 and the cathode I58. When the space current flow through the two tubes I65 and I56 is interrupted in the manner just explained, the voltage drops across the resistors I56 and I63 are reduced substantially to zero. Accordingly, the operating potential is removed from the screen electrode of the final amplifier tube I I I and this tube is rendered inoperatie to transmit signal currents to the hybrid coil I63. Also, the value of the negative bias impressed across the input electrodes of the amplifier tube I36 is decreased to normal so that this amplifier stage is conditioned to operate with normal gain. r

The manner in which the second control unit comprising the rectifier tube I88 and the discharge tube I16 functions partially or completely to, block the transmission channel I65 and to activate the final amplifier stage of the channel "I66 when'signal currents are transmitted from the line terminating at the terminals I66 through the hybrid coil I63 to the input side of the channel I 66, is substantially identical to the mode of operation of the first control unit as described above; More-particularly, signal currents transmitted to the input electrodes of the tube I36 through the coupling transformer I38 and the voltage divider I39 are amplified by this tube and the amplified signal voltage is impressed across the input electrodes of the two tubes I31. and I86. The. resulting space current fiow tion of the discharge tube I16, whereby a relatively heavy current is causedto traverse the control resistors I2I and I14. The voltage drop across these two serially connected resistors is utilized as an operating voltage for the screen electrode of the tube I31, whereby this tube is rendered operative to amplify the signal currents and transmit the same through the transformer I and the T connected volume control resistor network to the signal current coil of the loud speaker I02 for reproduction. The voltage developed across the resistor I2I is impressed as anegative bias between the input electrodes of the first amplifier tube I69 included in the channel I65, whereby this tube is either rendered com-:

pletely inoperative to transmit signal currents therethrough or the gain of the tube is reduced to a very low value. Shortly following the initial fiow of signalcurrents through the channel I66,

the slow-to-release relay I83, which is energized by the space current of the tube I86, operates momentarily to. extinguish the flow of'space ourrent through the discharge tube I16 with the result that the channel I66 is momentarily blocked and the amplifier I69 is momentarily recondi tioned to operate with-normal gain in the manner described .above with reference to signal transmission through the channel I65. During the transmission of signal currents through the channel I 66 to the loud speaker I62, the T connected resistance network included in this channel between the coupling transformer I and the signal current coil of the loud speaker opcrates in a well-known manner to maintain the sound output level of the loud speaker substantially constant regardless of changes in the level of the signal currents transmitted-to the input side of the channel. Whensignal current transmission through the channel I 66 is arrested the flow of space current through the rectifier tube I86 is arrested. 9 Thus, the operating winding of the relay I83 is deenergized and the potential impressed upon the start electrode I13 01' the discharge tube I 16 is restored to a negative value which'will-not permit'ioniration of the tube after the tube has been deionized. After an interval equal to the release period of the relay I83, the armature I 84 is moved from its attracted position to itsretracted position. During such movement.

the output circuit of the tube I16 isinterrupted,

thereby to disconnect the source of anodevoltage As a result, the tube is de-, of the space current path thereof is restored to the negatively biased start electrode I13. When the flow of space current through the two resistors Ill and- "Ms arrested the potential impressed upon the screen electrode of the amplifier tube I31 is reduced substantially to zero, whereby this tube is rendered inoperative. Also, the voltage drop across the resistor III is ionized and control sufiiciently reduced to permit the amplifier tube I69 of the channel I to tially normal gain.

The purpose of providing the relays I68 and I83, having slow-to-release characteristics, for controlling the output circuits of the tubes I56 and I16 respectively is to'prevent clipping. More particularly, the period or interval required for operate with substanthe release of either of these two'relays prevents the blocking of the channels during momentary pauses in signal current transmission through the respective associated channels. Tothis end, the relay I68. for example, provides a hang-over period, during which the discharge through the through the'rectifier tube I88 causes the ioniza (5 tube I56is sustained, which exceeds the short ml. of the amplifier stage comprising the tube I36 is pauses occurring in normal speech. Similarly, the slow-to-releas'e period of the relay I83 serves to sustain the flow of space current through the discharge tube I'I0--ior intervals during which short speech pauses may occur.

A modified arrangement for obtaining the delay intervals just described is illustrated in Fig. la. The relay arrangement illustrated in this figure may be used in lieu of the relays I68 and I83 as shown in Fig. 1 without changing the armature and contact spring assemblies. In the Fig. la arrangement the relay coil comprises a pair of differentially related windings, the lower winding of which is connected in series with a condenser I90 and a'resistor I8I across the upper windings. Also, the lower winding is constructed to provide a substantially greater number of ampere turns than the upper winding. Withthis arrangement included in the circuit of Fig. 1 the initial rush of current through. the upper winding occasioned by a flow of space current through the tube I65, for example, causes the relay armature rapidly to be attracted. During such energization, the condenser I90 charges relatively slowly so that the flow of current through the lower winding is relatively lowand no substantial opposing action occurs. When the flow of current through the upper winding is decreased or completely arrested,the condenser I 90 --discharges through the two windings-of the relay in series so that the ampere turns represented by the two windings are in aiding phase relation and the armature of the relay is maintained in its attracted position for a short time interval after the current fiow through the associated space current path ceases.

If the circuit-constants of the circuit illus trated in.Fig. 1 are so proportioned that each of the first amplifier stages respectively comprising the tubes I09 and I36 is not completely blocked when the complementary transmission channel is active, the arrangement permits what is known as conversation break-ins to occur. In this regard it will be apparent that signal currents transmitted to the substation circuit over the line connected to the terminals I00 are passed through the hybrid coil I03 to the input side of the channel I06 at a reasonablyhigh intensity as compared with. the currents transmitted from the channel I05 to the channel I06 through the hybrid coil during operation of the microphone With the circuit so arranged that the gain only reduced to a low value during signal transmiss io'n through the channel I05, the incoming signal currents are transmitted at reduced gain through this first amplifier stage of the channel I 06'5and arerimpressed across the input electrodes of the rectifier tube I80, whereby the discharge tube; "0 is ionized in the manner previously explained. In response to the flow of space current through the discharge tube I'| 0 the required operating voltage is impressed upon the screen electrode of the amplifier tube I3I, thereby to render thistube operativeto transmit the incomingsignal currentsto the loud speaker I02 for reproduction. The activation of the amplifier stage comprising the. tube I01. is accompanied by laf'decrease in the gain of the amplifier stage comprisingthetube Ina-r01- reasons which will be apparent "from the preceding explanation. After the initial conversation break-in, the user, of the illustrated substation circuit is expected to cease speaking, whereby the control unit comprising Thus, the control section .comprising the rectifier break in upon a speech train being transmitted to the circuit by speaking into the microphone IM to cause the activation of the channel I05 and a decrease in the gain of the amplifier tube I36, providing the circuit constants of the con- .trol circuit I are chosen so that the first amplifier stage of the amplifier I08 is not completely blocked when signal currents are being transmitted through the channel I06.

While it will be understood thattubes of any desired type having the proper operating characteristics may be used in the circuit illustrated in Fig. 1, preferably, the amplifier tubes I00, H0 and I36 are of the type commercially known as the 6J7G tube; the amplifier tubes III and I31 are of the type known as the 25A6G tube; the rectifier tubes I and I are of the type known a the 605G tube;-and the gaseous discharge tubes I56 and I10 are of the type commercially known as the 884 tube.

The arrangement illustrated in Fig. 2 of the drawingsis substantially similar to that shown in Fig. 1, but diners therefrom in that a difierent method is employed for controlling the gain of the amplifier stages included in the respective channels and a difierent type of relay arrangement is utilized for controlling the deionization of the two gaseous discharge tubes embodied therein. Due to the similarity between the two circuits, corresponding elements thereof have been identified by reference characters having the same tens and units digits but different hundreds digits respectively corresponding to the two figures. In the arrangement 01' Fig. 2, the first stage of amplification 209, included in the channel 205 is diagrammatically illustrated and the output electrodes of the last amplifier tube included in'this channel are coupled to the outgoing signal current terminals of the hybrid coil 203 through a coupling transformer 285. The control circuit 255 illustrated in Fig. 2 differs from the corresponding circuit I55 illustrated in Fig. 1 inthat each oi! the two relays 268 and 283 provided for controlling the deionization of the two discharge tubes 256 and 210, respectively, are double wound relays, the extra winding being arranged to be short-circuited in response to energization of the relay, whereby an additional slow-to-release period is imparted thereto. The two'control sections of the circuit are each arranged to control the gain of one amplifier stage in each of. the two signal transmission channels.

tube 265 and the discharge tube 256, which is coupled to respond to signal currents traversing the channel 205, is arranged to control the bias .voltage impressed between the input electrodes of the first'amplifier tube 238 included in the channel 206 and-the bias voltage impressed betweenthe inputelectrodes of the amplifier tube H I included in the channel 205. More particularly, signal voltages developed across the channel 205- at a point following the amplifier tube 2I0 are impressed upon the input electrodes of the rectifier 265 through a coupling condenser 286- This tube is normally biased to operate as a half wave rectifier by the voltage of the C battery 261 which is connected between the input elec-.

provided in the cathode leg of the output circuit of the rectifier 265 a biasing network which comprises a resistor 281 shunted by a signal current by-pass condenser 266. The load resistors 263 and 266 included in the output circuit of the discharge tube 256 are arranged respectively to control the gains through the amplifier tubes 2| I and 236. More particularly, the junction point between these two resistors is grounded and the negative terminal or the anode current source for the tube 256 is connected to the right terminal of the resistor 256. During space current flow through the discharge tube 256 the voltage drop developed across the resistor 263 is impressed through a resistor 269 between the input electrodes of the tube 2 in a direction to' overcome the normal negative bias afiorded by the C battery 296, thereby to increase the gain of the tube 2. Thevoltage developed across the resistor 256, on the other hand, is impressed through a filter network which comprises a resistor 29! and a condenser 292, and the voltage divider 239 between the input electrodesor the amplifier tube 236 in a direction to decrease the gain of the lastmentioned tube. In a similar manner, the control voltages developed across the resistors 22! and 216 when space current traverses the discharge tube 216 are utilized reversely to vary the gains through the amplifier tubes 2| 6 and 231, the voltage developed across the resistor 22| being impressed in a negative sense between the input electrodes of the tube 2l6 through a filter network which comprises a resistor 293 and a and the voltage developed across the resistor 214 being impressed in a positive sense between the input electrodes oi the tube 231 through a resistor 295; As illustrated in the drawings, the rectifier tube 266 is coupled through a condenser 26l to be controlled by signal currents traversing the channel 266, the input electrodes of this tube being biased substantially to the anode current cut-ofl. point through the provision of a source oi bias voltage 282 which is connected across the input electrodes of the tube 286 in series with a resistor 268. In this unit the control voltage for overcoming the bias voltage of the battery 211, thereby to condition the discharge tube 216 for ionization, is developed across a resistor 291 which is included in the cathode leg of the tube 266 and is shunted by a smoothing condenser 298.

The mode of operation of the arrangement illustrated in Fig. 2 is, in general, substantially described above with reference to similar to that the arrangement shown in Fig. 1. More particularly, with the cathode heater circuits of the various tubes illustrated in Fig. 2 energized the amplifier stage 269 and the amplifier stages re spectively comprising the tubes 2l6 and 236 are conditioned to operate with normal gain. The two amplifier tubes 2 and 231, on the other hand, are normally biased beyond cut-ofi by the voltages of the batteries 296 and 296 respectively. Thus, the two channels 265 and 266 are normally completely blocked so that the system is positively prevented from oscillating. With the apparatus in this condition, if sound waves impinge upon the diaphragm of the microphone 26I, the

output circuit of the rectifier resulting signal currents are amplified the amplifier stage 269 and are delivered to the input circuit of the second amplifier tube M6. The amplified signal voltages appearing across the output circuit of this second tube are impressed across the input electrodes of the third amplifier tube 2| I and also across the input electrodes of The resulting space curthe rectifier tube 265. rent flow through the tube 265 causes a voltage to be developed across the resistor 281 which reduces the negative potential upon the start electrode 256 of the discharge tube 256 to a point where this tube becomes ionized and current flows through the two load resistors 256 and '263.

The voltage developed across the resistor 263 bpposes the voltage of the 0 battery 266 and is of such magnitude that the .bias between the induced to permit this tube pressed as a negative put electrodes of the tube 2 is sufilciently reto operate with normal gain. Accordingly, the amplified signal currents delivered to the input electrodes of the amplifier tube 2 are amplified therein and transmitted through the coupling transformer 285 and the hybrid coil 263 to the line extending to the terminals 266. across the resistor 256, on the other hand, is imbias voltage between the input electrodes of the amplifier tube 236, thus reducing the gain of this tube. By suitably proportioning the resistance value of the resistor 256, the voltage developed .thereacross when space current traverses the discharge tube 256 may be proportioned to bias the tube 236 beyond cut-off such that the channel 266 is completely blocked, or to bias this tube so that it operates with only a small gain, whereby only signal ourlay of Fig. l, momentarily to break rents of substantial magnitudes may be transmitted therethrough. By thus reducing the gain of the amplifier tube 235, signal currents which may be transmitted through the hybrid system 263 to the input side of the channel 266,'due to an unbalanced condition ofthe hybrid system, are prevented from initiating the operation of the control unit comprising the rectifier tube 266 and the discharge tube 216. The relay 266 included in the output circuit of the rectifier 265 functions, in the exact manner previously explained with reference to the corresponding rethe space current path of the discharge tube 256 shortly following the initial signal current flow over the channel 265. During its operating period, therefore, the relay 268 momentarily blocks the transmission of signal currents through the channel 265 and increases the gain of the amplifier 236 to its normal value. When, however, the relay 266 is fully operated, the ionized condition of the tube 258 is reestablished, whereby the channel 265 is again rendered operative and the gain of the amplifier tube 236 is again reduced to an exceedingly low value. Upon operating, the relay 268 completes, at its armature 269, an obvious pathfor short-circuiting itsupper winding, thereby to render itselfexceedingly slow to release in a manner well understood in the art. By virtue of this arrangement, delay or hangover periods are provided to prevent the blocking of the channel 265 and the unblocking of the channel 266 during the short intervals which occur between speech syllables.

When the flow of signal currents through the sult, the relay 266 is deenergized and the poten-:

tial upon the start electrode 259 is restored to a The voltage developed value which will prevent space current flow through the tube 256 after this tube is deionized. After an intervaldetermined by the release period of the relay 266, this relay restores to interrupt, at its armature 263', the path shortcircuiting its upper winding, thereby to render 266 momentarily interrupts the The mode of operation of the control unit comv prising the rectifier tube 260 and the discharge tube 210 and associated with the channel 206 is identical with that just described with reference to the unit associated with the channel 205. From the foregoing explanation it will be ap-' parent that if the resistors 250 and 22| are properly proportioned so that the associated amplifier tubes 236 and H0 are not biased be.- yond cut-off when space current traverses these resistors, each of the two channels 205 and 206 is partially active when the other channel is in use. Accordingly, conversation break-ins may occur in a manner which will be apparent from the above description. In this regard it will be apparent that in the usual installation complete blocking of each channel when the other channel is in use is not required in order to prevent singing due to the substantial side tone suppression achieved through operation of the hybrid system 203.

1 Referring now more particularly to'Fig. 3 oi the drawings, the embodiment of the invention there illustrated is, in general, quite similar to the circuit shown in Figs. 1 and 2. Accordin ly. reference characters having the same tens and units digits but difierent hundreds digits have been used to identify corresponding elements of the three circuits. From an inspection of the circuit of the discharge tube 360 includes the resistor 361 and an additional resistor 312 which is shunted by a by-pass condenser 313. This latter resistor is connected in the manner illus-' the bias between the input electrodes of the amplifier tube 3| I. A

The rectifier unit 363 is similarly included in a circuit bridged across the output-circuit of the amplifier .tube 336. This circuit also includes a coupling condenser 314, a resistor 315 shunted by a smoothing condenser 316 and a second resistor 311 shunted by a by-pass condenser 316. The voltage developed across the resistor 315 when signal currents traverse the channel 306 is applied to the start electrode of the discharge tube 36l through a resistor 319 in opposition to the negative bias voltage normally impressed upon this electrode by a bias battery 360 through a resistor 30!. The voltage developed across the resistor 311 is utilized to'oppose the negative biasvoltage normally impressed across the input electrodes of the tube 331 by a bias battery 362 which is connected to the control grid of the indicated tube through a resistor 363. The output circuit of the discharge tube 36! includes,

in addition to the resistor 311, a second resistor Fig. 3 arrangement it will be apparent that the channel amplifiers 308 and 335 respectively included in the incoming and outgoing signal cur rent transmission channels 305 and 306 are substantially identical in arrangement with the corresponding amplifiers shown in Fig. 2. The control circuit 355 embodied in the arrangement'of Fig. 3 is, however, substantially diflerent from that shown in either Figs. 1 or 2. Briefly described, the control circuit 355 comprises a pair of control sections respectively associated with the two channels and individually including three-electrode gaseous discharge tubes 360 and 36l arranged respectively to be controlled by rectifier units 362 and 363. More particularly, a half wave rectifier circuit serially comprising sistor 365 shunted by a smoothing condenser 366 which is shunted by an alternating current by-pass condenser 365. Anode "current is supplied to the two discharge tubes 360 and 36i from a source of alternating current 366 having its output terminals coupled to the primary winding of a transformer 361 which includes two secondary sections 368 and 389 respectively included in the output circuits of the discharge tubes 360 and 36l.

With the circuit oi! Fig. 3 conditioned for operation, the two discharge tubes 360 and 361 aredeionized such that no current traverses the output circuits thereof. Under these conditions, the negative voltage impressed upon the control electrode 0! the tube 3 by the battery 316' serves to bias this tube beyond anode current cut-off. vSimilarly, the battery 362 normally'serves to bias the amplifier tube 331 bethe rectifier 362, a coupling condenser 36.6, a ;re-

366, and a resistor 361 shunted by a smoothing condenser 363, is connected across the'output circuit of the amplifier tube 310. The voltage developed across the resistor 365 when signal currents traverse the channel 365 is impressed through a resistor 366 between the input electrodes of the gaseous discharge tube 360 in opposition to the negative bias voltage normally impressed between these electrodes by a bias battery 310 through a resistor 31!. The output yond cut-oil. Accordingly, the two channels 305 and 366 are normally blocked to prevent the pas;-

sage of signal or noise currents therethrough, whereby local oscillation of the system is prevented. Since no current is traversing the output circuit of the tube 360, the voltage developed across the resistor 312 is relatively low; being of a value such that the amplifier tube 336 is conditioned to operate with normal gain. Similarly, the voltage across the resistor 366 is relatively low, being of a magnitude such that the amplifier tube 3" is conditioned to operate with normal gain. iwith the apparatus in ,this condition, it sound waves impinge upon the diaphragm of the microphone "I the resulting signal voltage as amplified by the two amplifier .stages 303 and tilt are impressed between the input electrodes of the amplifier tube 3 and across the above-traced rectifying circuit including the rectifier 362 and the resistors 365 and 361. The resulting voltage drop developed across the resistor 365serves sufiiciently to reduce the negative potential of the start electrode embodied in the discharge tube 360 to cause space current flow through this tube. By virtue of the alter dating character of the anode current supply path and the output circuit of the tube 360 only during alternate half cycles of the supply voltagefwhereby a pulsating direct current is caused to traverse the two resistors 361 and 312. This current is smoothed by the smoothing action of the shunt condensers 368 and 313, respectively. The direct voltage developed across the negative potential appearing at the left terminal of the resistor 312 is impressed through the voltage divider 339 upon the control .grid of the amplifier tube 336. Depending upon the resistance value of the resistor 312, this potential may serve to bias the amplifier tube 336 so that its gain is'verylow or to bias this tube beyond cutoff, thereby completely to block signal transmission through the channel 306. So long as signal currents traverse the rectifying circuit comprising the rectifier 362, the start electrode of the discharge tube 360 is maintained at a potential such that alternate ionization and de ionization of the discharge tube occurs during alternate half cycles of the anode current supply source. When, however, signal current transmission through the channel 305 is arrested, the flow of current through the rectifying circuit comprising the rectifier 362 ceases. Accordingly, the potential of the start electrode embodied in the discharge tube'360 is returned to its normal negative value. Thereafter and during the next cycle of the anode supply voltage when the potential of the discharge tube anode is reduced to zero tocause the deionization of the tube, the

start electrode'embodied in the tube is restored to control by virtue of the negative potential impressed thereon, thereby to prevent further space current flow through this tube. In response to the interruption of space current flow through the tube 360, the voltages across the resistors 312 and' 361 are reduced to zero or to very low values such that the amplifier tube 31! is again biased beyond cut-oil. and the gain of the amplifier tube 336 is restored to normal.

The mode of operation-of the other control unit, namely, that comprising the discharge tube 36! and the rectifier circuit comprising the rectifier unit 363, when signal currents traverse the channel 306 is substantially identical to the operation of the unit associated with the channel 305 as described above. Accordingly, it is believed thatthe'operation of the second control unit will be apparent without further explanation. It will further be apparent that if the resistance values of the resistors 312 and 384 are proportioned s thatthe tubes 336 and 310 are not biased beyond cut-off whenthe complementary signal current channels are in use, conversation break-ins may occur.

From the foregoing explanation it will beapparent that the voltages developed across the load resistors 312 and 384 and respectively utilized to control theamplifier tubes 336 and 3| 0 may have components of alternating current of the frequency of the source 386- included therein.

2,282,405 source 386', current traverses the space current I Such components would, of course, be delivered 7 through the respective associated channels to the output sidesthereof. In order toprevent such voltage components from producing interference, the source 386 is preferably constructed to have an output frequency well above the voice frequency range. For example, this source' may comprise an electronic oscillator having an output frequency which may range from five thousand to ten thousand cycles. Further, to prevent the voltage of this source from being amplified in the channel 306 and delivered to theloudspeaker 302 during periods when the indicated channel is in use, there is provided in this channel a low pass filter- 390 having an upper cut-off frequency of the order of five thousand cycles per second. With respect-to the components of current which may be transmitted from the anode current supply source 386 through the channel 305 and the hybrid system 303 to the line terminating at the terminals .300, if the fre'quency of the source 386 is suificiently high the attenuation over the lines connecting the circuit illustrated with a distant substation will be so great as to permit only a negligible portion of the currents of this frequency to be delivered to the reproducing means at the distant substation.

If. the load resistor 312 included in the output circuit of the discharge tube 360 ischosen of a value such that the amplifier tube 336 is not biased beyond cut-oil during signal transmission through the channel 305, signal currents may be transmitted from the output side of this channel through the hybrid coil 303 to the input side of the channel 306. The conditions of unbalance in the hybrid coil 303 might be such that these signal currents as'delivered through the tube 336 to the rectifier circuit comprising the rectifier 363 would initiate a disccharge through the tube 361, thereby to cause a decrease in the gain through the amplifier 308 and an increase in the gain through'the amplifier 335 in. the manner previously explained. In order to obviate this possibility the start electrode of the tube 36I may be connected to derive a portion of its bias potential from the output circuit of the tube 360. To this end, any de sired portion of the resistor 312 may be included in the biasing circuit for the start electrode of the discharge tube 36L the voltage'developed across this portion of the resistor 312 aids the bias voltage normally impressed upon the start electrode of the tube 36l by the battery 380. As a result, the voltage across the resistor 315 required to causeionization of the discharge tube 36| is substantially increased. By properly proportioning the constants of the circuit elements involved, this increase in the voltage required to trip thetube 36! may be sufiicient to prevent ionization of this tube in response to signal currents delivered from the channel 305 to the channel 306 through the coupling afforded by the hybrid coil 303, without being so great as to prevent the ionization of the tube 36l when signal currents are delivered to the input side of the channel 386 through the hybrid coil 303 fromthe line 3 connected to the terminals 300. A similar arrangement may be utilized to prevent inadvertent ionization of the tube 360 in the event the conditions imposed by a particular application require such an expedient.

While there has been described what is at present considered, to be the preferred embodi- As thus connected,

merit of the invention, it will be understood that various modifications may be made therein, and

it is contemplated to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a transmission system, a signal current determined start potential is impressed upon said start electrode, means for impressing said start potential upon said start electrode when signal currents traverseone of said channels, a signal current amplifier in the other of said channels, and means controlled by the resulting flow of current over said path for controlling the gain through said ampifier to change the signal ourcontrolled by the resulting flow of current over said path for controlling the gain through said amplifier to change the signal current transmission efficiency of said channel.

2. In a transmission system, a signal current transmission channel, a signal current amplifier in said channel, a control circuit comprising a gaseous discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the fiow of current over said path, an output circuit serially including said space current path, a source of alternating current and an impedance element, whereby pulsating direct current is caused to traverse said output circuit onlywhen a predetermined potential is impressed upon said start electrode, means for impressing said predeterminded potential upon said start electrode when signal currents traverse said channel, and means controlled by the voltage developed across said impedance element for controlling the gain through said signal current amplifier to change the signal current transmission efficiency of said channel.v

3. In a transmission system, a signal current transmission channel, a signal current amplifier in said channel, a control circuit comprising a gaseous discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the fiow of current over said path, an output circuit serially including said space current path, a source of alternating current and an impedance element,

'whereby pulsating direct current is caused to traverse said output circuit only when a predetermined potential is impressed upon said start electrode, rectifying means coupled to said channel and including an output circuit for impressing said predetermined potential upon said start electrode when signal currents traverse saidchannel, and means controlled by the voltage developed across said impedance element for controlling the rent transmission efiiciency of the other of said channels.

5. In a transmission system, a pair of channels for transmitting signal currents in different directions, a control circuit comprising a gas disruptive discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the flow of current over said path, an output circuit serially including-said space current path, a source of alternating current and an impedance element, whereby pulsating direct current is caused to traverse said output circuit only when a predetermined potential is impressed upon said-start electrode, means for impressing said predeterminded potential'upon said start electrode when signal currents traverse one of said channels, a signal current amplifier in the other of said channels, and means controlled by the voltage developed across said impedance element for controlling the gain through said amplifier to change the signal current transmission efficiency of the other of said channels.

6. In a transmission system, a pair of channels for transmitting signal currents in different directions, a control circuit comprising a gas disgain through said signal current-amplifier to change the signalcurrent transmission efliciency of said channel.

4. In a transmission system, a pair of channels for transmitting signal currents in diflerent directions, a control circuit comprising a gas disruptive discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the fiow of current over said path, a source of alternating voltage for delivering current to said space current path, whereby space current flow over said .path is automatically maintained only so long as a preruptive discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the fiow of current over said path, an output circuit serially including said space current path, a source of alternating current and an impedance element, whereby pulsating direct current is caused to traverse said output circuit only when a predetermined potential is impressed upon said start electrode, rectifying means coupled to one of said channels and including an output circuit for impressing said predetermined potential upon said start electrode when signal currents traverse one of said channels, a signal current amplifier in the other of said channels, and means controlled by the voltage developed across said impedance element for controlling the gain through said amplifier to change the signal current transmission efficiency of the other of said channels.

. 7. In a transmission system, a pair of channels for transmitting signal currents in different directions, a control circuit comprising a gas disruptive discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the fiow of current over said path, an output circuit serially including said space current .path, a source of alternating current and a pair of impedance elements, whereby pulsating direct current is caused to" traverse said output circuit only when a predetermined potential is impressed upon said start electrode, rectifying means coupled to said channel and including an output circuit for impressing said predetermined potential upon said start electrode when signal currents traverse one of said channels,- means controlled by the voltage developed across one of said impedance elements for increasing the signal current transmission efficiency of said one channel, and means controlled by the voltage developed across the other 01 Said impedance elements for decreasing the signal current transmission eiiiciency of the other-of said channels.

8. Telephone substation apparatus comprising transmitting means, receiving means and a hy-- signal current transmission efliciency of said one channel and decreasing the signal current transmission efliciency oi the other channelwhen signal currents traverse said one channel, means comprising a three electrode gaseous discharge tube having input electrodes coupled to said other channel and output electrodes coupled to both of-said channels for simultaneously increasing the signal current transmission efliciency of said other channel and decreasing the signal current transmission efliciency of said one channel when signal currents traverse said other channel, and means comprising a source of alternating current commonly coupled to the output e1ectrodes of each or said tubes for extinguishing the space current flow through each of said tubes each'time the flow of signal currents through.

the respective associated channels is arrested.

ROSWELL H. HERRICK. 

